Aspects of the present invention relate to a semiconductor device cooling module and, more particularly, to a separable and replaceable cooling module for cooling one or more semiconductor devices.
In the packaging of semiconductor chips, an organic substrate is typically used to fan out fine pitch (e.g., 0.15 to 0.2 millimeter (mm)) controlled collapse chip connection (C4) solder bumps on the silicon die to larger pitch (e.g., 1.0 to 1.2 mm) ball grid array (BGA) or land grid array (LGA) connections. With a BGA, the chip package is attached to a printed circuit board by reflowing solder balls to form a permanent connection whereas an LGA type interposer provides a connection where the chip package can be readily removed and replaced on the printed circuit board (PCB). An LGA interposer may be a “hybrid-type” where the bottom of the interposer is connected to the PCB by BGA solder balls, but the upper connection to the substrate is through an LGA type connection. No distinctions will be made between an LGA and hybrid LGA interposers.
In general, with an organic packaging substrate, a lid formed from a thermally conductive material such as copper is attached to the chip and the organic substrate to protect the chip during handling and add mechanical strength to the organic substrate. A thermal interface material (TIM) material is dispensed between the back surface of the chip and the lid to provide a thermal path for heat dissipation. If required, a heat sink is then attached using a second TIM layer to the outside surface of the lid, although the need for first and second TIM layers can be an unacceptable limitation in some cases.
The chip is mounted face or device side down on the packaging substrate. When used with an LGA interposer, a compressive load is applied to the package lid to make electrical contact between the LGA and the PCB. This compressive load may be applied either in a center position above the chip or at two or more points on a perimeter of the package lid.
For high performance computing, there has recently been significant development on various chip stack structures as further improvements to device performance that are obtained by scaling down device dimensions is becoming increasingly difficult to achieve. For example, in some chip stacks, the chips are already thinned down to enable the fabrication of fine pitch thru silicon vias (TSV), which can reduce the mechanical strength of the chips. Therefore, it may be desirable to avoid actuating the load for an LGA through the chip stack, especially as the size of the substrate and hence the required load increases.
For applications with a high power density or which require a low junction operating temperature, a packaging solution where the heat sink can be directly attached to the back surface of the chip or chip stack may be required. The use of only a single TIM layer between the chip or chip stack and the heat sink results in improved thermal performance compared to a lidded chip package where two TIM layers are required. This is typically referred to as a lidless or direct heat sink attach package. For high performance systems, it is generally desirable to use LGA chip packages instead of BGA chip packages so that the chip can be replaced if necessary. As the chip complexity increases and the power and input/output (I/O) requirements grow, the size of the package generally increases to provide a greater number of LGA contacts.
For a lidless package where the LGA actuation load is provided through the chip, the substrate or the combination of the substrate and top surface stiffener, there may be a need to provide enough mechanical rigidity to distribute a load with sufficient uniformity across the LGA interposer to facilitate formation of electrical contacts for all the connection pads. For organic substrates, this may limit an allowable substrate size and for ceramic substrates this may increase the required thickness. When the LGA actuation load is provided to the substrate, a load frame (or stiffener) with an opening for the chip may be attached to the substrate, and the combined structure needs to provide adequate mechanical stiffness to actuate the LGA uniformly.
A further trend in high performance chip cooling is the use of direct water cooling where a water cooling device is attached to the chip with a TIM layer. This can enable a lower junction temperature for improved reliability, allow for higher power density chips and improve overall system energy efficiency. Generally, with field replaceable water cooled modules, either a TIM layer or water connections need to be broken or reworked to replace a module. To avoid any risk of spilling water or subsequent leaks, reworking the TIM layer is the preferred approach. As noted above, LGA interposers are used for field replaceable modules.